The invention is based on a fuel injection system for internal combustion engines as defined hereinafter. In one such known fuel injection system, in which for optimal fuel combustion in the engine combustion chamber, in addition to free control of the injection onset and end, the injection pressure can also be regulated freely, a high-pressure fuel pump feeds fuel from a fuel supply tank at high pressure via a feed line to a high-pressure collection chamber, which communicates via high-pressure lines with the various injection units, which correspond in number to the injection locations in the combustion chamber of the engine to be supplied. These injection units each comprise one injection valve, protruding into the engine combustion chamber, and one three-way valve controlling it; the valve member of the injection valve, which is in the form of a seat valve, has on its shaft a pressure face formed by a cross-sectional reduction in the direction of the valve seat, and with this face it protrudes into a first pressure chamber, which communicates continuously with the high-pressure line to the high-pressure collection chamber and with the injection port at the valve seat, and whose pressure acts upon the valve member in the opening direction. With its end remote from the valve seat, the valve member defines a second pressure chamber, which can be made to communicate via the three-way valve with either the high-pressure line or a relief line to the fuel tank; the effective cross section of the valve member acted upon by the pressure is less in the region of the first pressure chamber than in the region of the second pressure chamber.
Furthermore, a check valve opening in the direction of the second pressure chamber and a throttle in the line parallel to it are disposed between the second pressure chamber of the injection valve and the three-way valve, which is triggered by an electric control unit.
The injection process in the known fuel injection system is controlled as a function of engine operating parameters, by means of the three-way valve, which in the intervals between injection connects the second pressure chamber at the injection valve to the high-pressure line, so that the fuel pressure acting upon the larger end face keeps the valve member closed, counter to the opening force of the pressure face in the region of the first pressure chamber. If injection is to occur, then the three-way valve switches over and connects the second pressure chamber with the relief line, thus pressure-relieving it, and the fuel pressure in the first pressure chamber, acting upon the valve member in the opening direction, now suffices to lift the valve member from its seat. To avert overly rapid opening of the injection valve, with the attendant high injection rate, at the injection onset, the fuel flowing out of the second pressure chamber is throttled at the throttle restriction, so that the opening motion is slowed down and at first only a small quantity of fuel is injected into the combustion chamber and can then be prepared for optimal combustion. The end of injection is controlled by reconnecting the second compression chamber of the injection valve with the high-pressure line, following which the high pressure rapidly builds up in the second pressure chamber and thus moves the valve member back into its closing position.
However, the known fuel injection system has the disadvantage that the shaping of the injection course, particularly at the onset of injection, is done via a throttle restriction provided in addition to the three-way valve and via an additional check valve; these elements require additional space and involve additional production costs.